virshStreamSkip: Emulate skip for block devices

virshStreamSkip: Emulate skip for block devices

This callback is called when the server sends us STREAM_HOLE
meaning there is no real data, only zeroes. For regular files
we would just seek() beyond EOF and ftruncate() to create the
hole. But for block devices this won't work. Not only we can't
seek() beyond EOF, and ftruncate() will fail, this approach won't
fill the device with zeroes. We have to do it manually.

Partially resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1852528

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>